In recent years, in order to satisfy requirement for enhancing performance of semiconductor devices, it has been proposed to use a semiconductor wafer in which a Si1-XGeX layer (0<X≦1) is epitaxially grown on a surface of a silicon single crystal. For example, a semiconductor device such as a high-speed MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor) in which the Si1-XGeX layer is used in a channel region has been proposed.
In this case, because Si1-XGeX crystal has a larger lattice constant than that of Si crystal, strain (compression strain) is generated in a crystal lattice of the Si1-XGeX layer that is epitaxially grown on a surface of a silicon single crystal. By its strain stress, an energy band structure of the Si1-XGeX crystal is changed and therefore, degeneracy of the energy band is solved and an energy band having a high carrier mobility is formed. In the Si1-XGeX layer having strain, particularly, its hole mobility becomes high and therefore, high-speed operating characteristics are expected in the MOSFET in which the Si1-XGeX layer is used in a channel region. Here, the Si1-XGeX layer has a Ge concentration of more than 0% and contains a Ge layer having a Ge concentration of 100% and hereinafter, will be occasionally described simply as SiGe layer.
As a method for forming such a SiGe layer, there has been disclosed an oxidation enrichment method in which a SiGe layer is epitaxially grown on an SOI (Silicon On Insulator) wafer and then, its surface is thermally oxidized in an oxygen atmosphere and thereby, Ge is enriched and a SiGe layer having a high Ge concentration is formed (See, 51th Applied Physics Related Association Lecture Meeting, Lecture Abstracts, 28p-ZZ-6, p. 22, 51th Applied Physics Related Association Lecture Meeting, Lecture Abstracts, 30a-YL-10, p. 414). It has been supposed that according to this method, a SiGe layer having a high Ge concentration and a thin thickness can be formed.
Hereinafter, such a wafer in which a SiGe layer is formed on an insulator will be occasionally described as an SGOI (SiGe On Insulator) wafer and, a wafer in which a Ge layer having a 100% Ge concentration is formed will be occasionally described as a GOI (Ge On Insulator) wafer.